Introduction to HVDC.pdf

The various advantages of hvdc transmission systems

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The various advantages of HVDC transmission systems over conventional ac transmission lines are summarized below: Transmission of large amounts of electrical power over distances exceeding 1000 km without the need for voltage compensation along the line. The ac power systems linked by an HVDC transmission system do not need to operate in synchronism (they can even operate at two different frequencies). These networks can thus preserve their operational independence. No practical technical limit to the length of submarine transmission of electrical power. Flexible control allowing fast and precise control of the amount of electrical power transmitted between the linked ac power systems. Also, the direction of power flow can be reversed as required. Transmission line generally consists of only two conductors, requiring a smaller right of way and less obtrusive towers. Main components in a converter station of an HVDC transmission system Figure 4 shows the main components in a converter station of an HVDC transmission system. Each component is described below. AC circuit breaker : isolates the HVDC transmission system from the ac power network when the HVDC transmission system does not operate properly. AC filters : reduce harmonics at the ac side of the converter, and thus help in mitigating the undesirable effects of these harmonics in the ac power network. Capacitor bank : provides reactive power to compensate for the reactive power drawn by the converter station, thereby correcting (i.e., maintaining close to unity) the power factor of the HVDC transmission system.
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Introduction – HVDC Transmission Systems HVDC Transmission Systems 5 Figure 4. Simplified diagram showing the main components in a converter station of an HVDC transmission system. On-load tap changer (OLTC) transformers : multiple-tap transformers whose taps can be changed without interrupting power, in order to adjust the ac power network voltage at the level required for proper operation of the converter station. These transformers also provide isolation between the ac power network and the dc side of the HVDC transmission system. The configurations normally used for the OLTC transformers are the wye-wye (Y-Y) and wye-delta (Y- ) configurations. Thyristor converter : converts ac power to dc power for transmission over the dc line or, conversely, converts dc power from the transmission line to ac power. The thyristor converter is actually made of many thyristor three-phase bridges that are series connected in order to obtain the desired operating voltage. The thyristor bridges in a converter station are arranged in stacks which are often suspended from the ceiling of the building that houses the station. Smoothing inductor and dc filters : reduce harmonics at the dc side of the HVDC transmission system, limit current surges into the thyristor converter, help prevent current flowing through the thyristor converter from becoming discontinuous at low power levels, and improve the stability of the system.
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